High modulus fibers

Aromatic polyamide fibers are produced by spinning liquid crystalline polymer solutions of PPTA-sulfuric acid dopes into a water coagulation bath [337], resulting in the formation of a crystalline fiber with an exterior surface skin. 

Variations in the structure produced by annealing at elevated temperature are known to increase the fiber modulus due to a more perfect alignment of the molecules [371]. The chemistry and physics of the aromatic polyamide fibers have been reviewed [342]. 

The structure of the aramid fibers has been studied by Dobb and Johnson [363,372-376] 

Poly(p-phenylene benzobisthiazole) is one of a group of polymers with rod-like molecules, spun into high strength fibers as part of the Air Force Ordered Polymers Research Program [343]. Allen et al. [383] described the development of high mechanical properties from anisotropic solutions of the polymer. The structure-property 

Odell et al. [347] studied the role of heat treatment, which is known to enhance mechanical properties, using PPBT (model) films. The extruded films have a rather poorly developed crystallinity. On heat treatment the films change color from light straw to blue, and dark field EM studies show that the increase in modulus with heat treatment is likely due to the increase in 

In the past two decades, major technological developments have occurred in the production of polymer fibers with high mechanical strength and stiffness. Materials science studies have been directed toward a better understanding of the relationship among chemical composition, physical structure, and mechanical properties. 

Aromatic polyamide fibers fail in tension by axial splitting, resulting in fine fibrils that can be 

Strength, and internal helical cracks in a high modulus variant were shown to result in decreased tensile strength [683-685], Finally, the internal structure of aramid fibers has been studied by AFM methods (e.g., [631, 632, 686, 687]), which provides insights into the nanostructure. 

Poly(p-phenyiene benzobisthiazole) is one of a group of polymers with rod-like molecules, spun into high strength fibers. Allen et al. [688] 

Structural investigations have been carried out by many researchers, including Allen et al. [691], who used wide angle x-ray diffraction, mechanical testing, and SEM imaging of fractured as spun and heat treated PBZT fibers. 

The structure of the aramid fibers has been studied by Dobb and Johnson [447, 473-477] and summarized by many others [406,415,417-419, 478]. Dobb et al [473] first showed a lattice image for fibrillar fragments produced by sonica- 

Ultrathin sections of PBZT on TEM grids were used for an OM study. Optical micrographs are shown of an extruded film section taken in polarized light (Fig. 5.104, color section). Selected area electron diffraction patterns were taken of the sections in the TEM in the core (Fig. 5.105A) and the skin (Fig. 5.105B). The diffraction results show a variation in orientation the core is much less oriented than the skin. This is consistent with the results of Minter et ah [492] who used 

Thermotropic aromatic copolyesters have a major advantage over the lyotropes as the thermo- 

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Properties. As prepared, the polymer is not soluble in any known solvents below 200°C and has limited solubiUty in selected aromatics, halogenated aromatics, and heterocycHc Hquids above this temperature. The properties of Ryton staple fibers are in the range of most textile fibers and not in the range of the high tenacity or high modulus fibers such as the aramids. The density of the fiber is 1.37 g/cm which is about the same as polyester. However, its melting temperature of 285°C is intermediate between most common melt spun fibers (230—260°C) and Vectran thermotropic fiber (330°C). PPS fibers have a 7 of 83°C and a crystallinity of about 60%. 

Eor the case of high modulus fibers such as carbon fibers with = 240 GPa (3.5 x 10 psi), in a polymer matrix, such as epoxy resin with = 3.0 GPa (450,000 psi), the extensional modulus is approximately proportional to the fiber volume fraction and the modulus of the fibers ... 

Today, carbon fibers are still mainly of interest as reinforcement in composite materials [7] where high strength and stiffness, combined with low weight, are required. For example, the world-wide consumption of carbon fibers in 1993 was 7,300 t (compared with a production capacity of 13,000 t) of which 36 % was used in aerospace applications, 43 % in sports materials, with the remaining 21 % being used in other industries. This consumption appears to have increased rapidly (at 15 % per year since the early 1980s), at about the same rate as production, accompanied by a marked decrease in fiber cost (especially for high modulus fibers). 

Poly(/)-phenylenctcrcphthalamiclc) forms a liquid crystalline solution and can be spun into a fiber with a very high orientation these fibers have excellent tensile and thermal properties. These high-modulus fibers are suitable as reinforcing materials in technical applications. 

We control fiber properties by changing the relative speeds of different stages of the process. Orientation is increased and fiber thickness decreased by increasing the final take-up speed relative to the rate at which the molten polymer strands leave the spinneret. To produce high modulus fibers we generally adopt conditions that maximize orientation. Fiber diameters... 

Aromatic poly(benzoxazole)s exhibit excellent thermal stability. Rigid-rod poly(benzoxazole)s are fabricated into high-strength and high-modulus fibers. Fluorine-containing aromatic poly(benzoxarole)s are expected to have unique properties. 

Dieffendorf, R. J. (1985). Comparison of the various new high modulus fibers for reinforcement of advanced composites with polymers, metals and ceramics as matrix, pp. 46-61. In Fitzer, E. ed. Carbon Fibers and Their Composites, Springer-Verlag, New York. 

ASTM Standards and Literature References for Composites Materials, Second ed. ASTM Committee D-30 on High Modulus Fibers and their Composites (1990) Philadelphia, ASTM... 

Both anisotropic dopes of BBL [41] and the pseudo ladder DIOH-PBZT [42] were dry-jet wet spun into high modulus fibers and their compressive... 

Mullin, J. V. Analysis of Test Methods for High Modulus Fibers and Composites, 349, ASTM STP 521, Philadelphia, PA (1973)... 

The study of thermotropic, as well as of lyotropic LC polymers is directly linked to a series of practical tasks, regarding the construction of polymeric materials with set properties. For instance, making use of anisotropy of the LC state in processing (particularly in moulding) of polymeric materials discloses impressive prospects for the production of so called high modulus fibers and films 18 25). 

Ultra-high modulus fibers such as aramid and carbon fibers have been currently utilized for composite material fabrication. Ultra-high modulus polyethylene (UHMPE) fiber is also applicable for composite fabrication because of the light weight in addition to its high modulus, vibration damping, and resistance to chemicals. However, this fiber has drawbacks such as poor interfacial adhesion with the polymer matrix of the composite because of highly hydrophobic nature of the fiber surface. 

Structure of High Modulus Fibers of Poly-jJ-Phenylene Benzbisthiazole... 

As already illustrated by a previous paper (5 ) on the structure of high modulus fibers, electron microscopy can be very successful when applied to these beam-resistant materials, and so constitutes an essential complement to x-ray studies. In the present work, electron diffraction coupled with BF and DF imaging has allowed detection of the best ordered zones within PBT fibers which illustrates the structure possibly obtainable by fiber processing refinement. The well ordered structures observed thus far compare rather well, with the exception of their fibrillar texture, to the structure of PPT high-modulus fibers. The two dimensional character of the crystallites is likely due to the freedom of axial translation of the molecules. Future work should determine if this feature is a direct consequence of the chemical structure of the PBT molecule or is simply the result of non-optimized processing conditions. 

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